Flexible substrate, Multilayer flexible substrate and process for producing the same

ABSTRACT

A flexible substrate comprising: (i) a film; (ii) an insulating resin layer formed on each of a front face of said film and a rear face of said film which face is opposite to said front face; (iii) a front-sided wiring pattern embedded in the insulating resin layer formed on said front face of said film, and a rear-sided wiring pattern embedded in the insulating resin layer formed on said rear face of said film; and (iv) a via which is located between a front-sided wiring pattern and a rear-sided wiring pattern and serves to electrically connect between said front-sided wiring pattern and said rear-sided wiring pattern; wherein said insulating resin layer formed on each of said front face and said rear face of the said film is thicker than said film.

FIELD OF THE INVENTION

The present invention relates to a flexible substrate. In particular, the present invention relates to a flexible substrate and a multilayer substrate which are suitable for mounting, and also relates to a process for producing the same.

BACKGROUND OF THE INVENTION

A flexible substrate (i.e. flexible printed circuit) has a fundamental structure composed of a conductor and a heat-resisting polymer film. The flexible substrate in which the conductor is disposed only on one side of the heat-resisting polymer film is called “single-sided flexible substrate”. In contrast, the flexible substrate in which the conductors are disposed on both sides of the heat-resisting polymer film is called “both-sided flexible substrate”.

A copper clad laminate (CCL) is generally used to produce the single-sided flexible substrate. There are two kinds of copper clad laminates, wherein one is a three-layer CCL in which a copper foil is disposed on the heat-resisting polymer film via an adhesive, the other is a two-layer CCL in which a copper foil is disposed on the heat-resisting polymer film without an adhesive layer. Such two-layer or three-layer CCLs are respectively produced by means of a laminating process, a casting process or a sputtering/plating process. A subtractive process is carried out by using of the two-layer or three-layer CCLs, and thereby the single-sided flexible substrate is consequently obtained.

The laminating process of the single-sided or both-sided flexible substrates by using of a film and an insulating resin layer leads to a multilayer flexible substrate. The through-hole conductor (i.e. via) is formed in multilayer flexible substrate by coating the inner wall of the through-hole with a metal. Therefore, the wiring patterns of respective layers in the multilayer flexible substrate are electrically connected to each other.

Such flexible substrate or multilayer flexible substrate can be effectively used for a spatially narrow mounting-area due to a flexibility thereof. For example, The flexible substrate or multilayer flexible substrate are mounted for not only a small space around a compact liquid crystal provided for in a camera, a cell-phone or a portable PC, but also a small space around a PC peripheral equipment such as a printer or a HDD. Recently, a further high density and performance of the semiconductor has been required as the electronics device is becoming more compact, lighter and thinner. Therefore, the flexible substrate having the semiconductor or a passive element is also required to be thinner and high-density. For example, not only an increase of the output terminal number, but also a fine pad pitch of the driver IC is required with a progress in a colorization and high definition of a liquid crystal display.

With respect to the foregoing, the related documents are as follows:

-   -   Japanese Patent Kokai Publication No. 11-157002 (see 3 page         thereof);     -   Japanese Patent Kokai Publication No. 2004-31588 (see 2 page         thereof);     -   Japanese Patent Kokai Publication No. 4-107896 (see 1 and 2         pages thereof);     -   Japanese Patent Kokai Publication No. 2-180679 (see 1 page         thereof);     -   Japanese Patent Kokai Publication No. 10-256700 (see 2 and 3         pages thereof);     -   Japanese Patent Kokai Publication No. 2000-77800 (see 1 page         thereof);     -   Japanese Patent Kokai Publication No. 2003-224366.

A conventional flexible substrate and a conventional process for producing the same were fraught with problems as shown in the following matters (I)-(VII):

-   -   (I) The miniaturization of the wiring pattern is important from         the standpoint of producing a thinner and high-density flexible         substrate. There is a limitation of a fine wiring of the wiring         pattern as for a subtractive process (i.e. chemical etching)         because a thickness of the copper foil used in the flexible         substrate is from 18 to 35 μm. That is to say, it is difficult         for the subtractive process to produce a less than and equal to         75 μm of the line width of the wiring pattern by using of the         copper foil having a thickness of 18-35 μm. As a result, a         thinner copper foil is needed for attaining a further         miniaturization of the wiring pattern.     -   (II) In the case where the subtractive process such as a         chemical etching is used to produce a wiring pattern, there is a         possibility that an etchant is left behind between the wiring         patterns, which in turn adversely affects a reliability of the         electrically insulation. The subtractive process gives such a         construction that a wiring pattern protrudes from the surface of         the substrate. The protrusion of the wiring pattern results in a         decrease of the surface flatness in the substrate. Thus, there         is a possibility that a bump provided in the semiconductor chip         is somewhat difficult to be mounted on the wiring pattern. And         also there is a possibility that the mounted bump is moved into         between the wiring patterns, and therefore a short-circuit         occurs. Also, the protrusion of the wiring pattern itself may         cause to interrupt a plastic molding process that is afterward         carried out.     -   (III) The through-hole conductor is generally used to connect         the wirings of the respective layers to each other. This         indicates that an increase in the number of the layers will lead         to an increase in the number of the through-hole conductor,         which in turn will make less sufficient space for the wiring.         Therefore, it is a general method to laminate the single-sided         flexible substrates provided with the through-hole conductors or         the both-sided flexible substrates provided with the         through-hole conductors. In this case, the through-holes are         filled with a metal paste. Such metal paste inevitably contains         a liquid resin or solvent in terms of an efficient filling and         printing, and thus the resulting circuit has a higher resistance         than that of a conventional circuit prepared by a copper plating         process. With a decrease in diameter of the through-hole, the         through-holes become increasingly harder to be filled with the         metal paste. Thus, a viscosity and fluidity of the metal paste         are required to be adjusted by adding a large amount of the         solvent to the metal paste. This results in an evaporation of         the solvent contained in the metal paste, which in turn leads to         a formation of gas cavities. As a result, the resistance of the         through-hole itself increases due to the gas cavities.     -   (IV) In the case where the through-hole conductor is formed, an         adhesive layer and a film are respectively perforated by means         of the laser machining. The adhesive layer is easy to be         machined with the laser, while on the other hand, a thick film         used in a conventional flexible substrate is difficult to be         machined with the laser. Concretely, the hole obtained by         laser-machining a conventional organic film is not circular in         shape due to the heat during the laser machining process, which         in turn leads to the burr. Also, the metal paste is difficult to         be poured into the hole due to the fact that the diameter of the         incident laser is smaller than that of the outgoing laser.     -   (IV) The fine wiring pattern or through-hole conductor as well         as a thin circuit component is important from the standpoint of         producing a thinner and high-density flexible substrate. Passive         elements such as an inductor, a condenser or a resistor are         generally mounted on the surface of the substrate in such a         manner that they protrude from the surface of the substrate.         This will cause such a problem that the substrate becomes thick         as a whole.     -   (VI) In a conventional flexible substrate, a passive or an         active element is formed on the exposed surface of the         substrate, so that the passive or the active element is not         included within the flexible substrate. Thus, in the case where         the multilayer substrate are produced by using of such flexible         substrate, a multilayering process is carried out agaist the         passive or the active element formed on the exposed surface, and         consequently the passive or the active element is formed between         respective layers as well as between the wiring patterns.         Therefore, this will cause such a problem that the region for         the wiring pattern becomes smaller.

The flexible substrate is needed to be folded in a smaller space. Thus, a better flexing life (or sliding flexibility) of the flexible substrate is required. Therefore, a sufficient flexing life of the two-layer CCL is required and thereby a high adhesion strength between a polyimide film and a copper foil is also required. Further, in the case of the three-layer CCL, in addition to the above adhesion strength, a high adhesion strength among the polyimide film and the copper foil, and an adhesive composition is required.

Furthermore, the conventional wiring pattern formed by the etching process is exposed to its surroundings on surfaces of the flexible substrate. This will cause a microcrack in wiring patterns when the flexible substrate is folded, which will be far from satisfying in terms of the flexing life.

Considering the challenges or problems as described with respect to the aboves (I) to (VI), an object of the present invention is to provide a reliable, high density and thin flexible substrate that is better in terms of the flexing life. A further object of the present invention is to provide a process for producing such flexible substrate.

SUMMARY OF THE INVENTION

In order to achieve the object, the present invention provides a flexible substrate comprising:

-   -   (i) a film;     -   (ii) an insulating resin layer formed on each of a front face of         the film and a rear face of the film which face is opposite to         the front face;     -   (iii) a front-sided wiring pattern embedded in the insulating         resin layer formed on the front face of the film, and a         rear-sided wiring pattern embedded in the insulating resin layer         formed on the rear face of the film; and     -   (iv) a via which is located between a front-sided wiring pattern         and a rear-sided wiring pattern and serves to electrically         connect between the front-sided wiring pattern and the         rear-sided wiring pattern;     -   wherein the insulating resin layer formed on each of the front         face and the rear face of the film is thicker than the film.

In order to obtain such flexible substrate, the present invention provides a process for producing a flexible substrate comprising a film, an insulating resin layer and a wiring pattern, said process comprising the steps of:

-   -   (a) forming the insulating resin layer on each of a front face         of the film and a rear face of the film which face is opposite         to the front face wherein the insulating resin layer is thicker         than the film;     -   (b) making a through hole in the film and the insulating resin         layer;     -   (c) filling the through hole with a conductive resin         composition; and     -   (d) embedding a wiring pattern into each of the insulating resin         layers formed on the front face and the rear face of the film in         such a manner that the wiring pattern is electrically connected         to the conductive resin composition.

In the flexible substrate according to the present invention, the wiring pattern is buried in the insulating resin layer. Preferably, the wiring pattern is buried in the insulating resin layer in such a manner that the surface of the wiring pattern is on the same level as that of the insulating resin layer. As for the flexible substrate according to the present invention, the via is located between the front-sided wiring pattern and the rear-sided wiring pattern, so that no via penetrates wholly through the flexible substrate. Thus, such via is herein referred to also as “inner via”.

The flexible substrate according to the present invention is better in terms of a surface flatness because the surface of the wiring pattern is on the same level as that of the insulating resin layer. As a result, an extremely precise mounting is possible when the semiconductor chip is mounted to the flexible substrate. Also, the adhesion strength between the insulating resin layer and the wiring pattern buried therein is kept in the flexible substrate. Thus, the stress applied on the wiring pattern will disperse, which in turn will lead to a sufficient flexibility (i.e. better flexing life) of the flexible substrate. Furthermore, the wiring pattern is embedded into the insulating resin layer by means of a transferring technique, so that a clean surface of the substrate on which there is no residue (e.g. etchant) is obtained, which in turn will lead to a better reliability in an electrical isolation.

It is possible to arrange the via at an optional position upon producing the flexible substrate. This allows an electric continuity at a desired position of the wiring pattern, which in turn will facilitate a wiring design. Also, due to the buried wiring pattern, the spacing between the front-sided wiring pattern and the rear-sided wiring pattern becomes small, which in turn leads to a smaller via.

In the case where the multilayer flexible substrate is produced by using of the flexible substrate of the present invention, a passive or an active element is formed on the film. Thus, it is possible to mount the passive or the active element at a high density in such a manner that the length of their wiring is short. By combining a variety of passive and active elements to each other, a more high-performance electronic circuit can be included within the flexible substrate. Also, a high-density mounting of the passive and active elements helps to minimize an adverse effect on the circuit, which effect is caused by a parasitic capacity between the wirings as well as an inductance. In addition, a variety of the passive and active elements can be arranged inside the substrate, so that the area required for a surface-mounting as well as the number of the components is minimized, which in turn will lead to a smaller and thinner flexible substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-sectional view of a flexible substrate 100 according to the present invention.

FIG. 2 shows a schematic cross-sectional view of a flexible substrate 110 including a passive element according to the present invention.

FIG. 3 shows a schematic cross-sectional view of another flexible substrate 120 according to the present invention.

FIG. 4 shows a schematic cross-sectional view of multilayer flexible substrate 200 according to the present invention.

FIGS. 5(a) to (d) show cross-sectional views illustrating the steps in a process for producing a flexible substrate 100.

FIGS. 6(a) to (e) show cross-sectional views illustrating the steps in a process for producing a flexible substrate 110 including a passive element.

FIG. 7 is a graph that shows an elongation modulus of a film versus a flexing number.

FIG. 8 is a graph that shows a curvature radius versus a flexing number.

FIG. 9 is a graph that shows a ratio of an insulating resin layer thickness/a film thickness versus a flexing number.

Hereinafter, the flexible substrate of the present invention and the process for producing the same will be concretely described.

FIG. 1 shows a cross-sectional view of a construction of the flexible substrate 100 of the present invention. As shown in FIG. 1, the insulating resin layers 2 a, 2 b are formed on the film 1 wherein each of the insulating resin layer is thicker than the film, and the wiring pattern 3 a, 3 b are embedded into insulating resin layers 2 a, 2 b. The vias are located between the wiring pattern 3 a and the wiring pattern 3 b. Thus, these vias serve to electrically connect wiring patterns 3 a, 3 b to each other.

In the flexible substrate 100 of the present invention, the insulating resin layers 2 a, 2 b are formed in such a manner that each of them is thicker than the film 1. For example, a ratio of a insulating resin layer thickness to a film thickness (=insulating resin layer thickness/film thickness) is preferably 1.1 to 8, more preferably 1.2 to 6. As used in this specification and claims that follows, the term “insulating resin layer thickness” means a thickness of an insulating resin layer formed on the one surface of the film. Concretely, the thickness of each of the insulating resin layers 2 a, 2 b is, for example, 3 to 80 μm, and the thickness of the film 1 is 2 to 16 μm. In the case where each of the insulating resin layer 2 a, 2 b is thicker than the film 1, a flexing life or a sliding flexibility of the obtained flexible substrate 100 becomes improved. The reason for this is that, when the flexible substrate is folded, the stress applied on the film and the buried wiring pattern is alleviated by the insulating resin layer having a low modulus of elasticity.

Further, in the case where the wiring patterns 3 a, 3 b are embedded into the insulating resin layers 2 a, 2 b, the thickness of each of the wiring patterns 3 a, 3 b is preferably 40 to 100% of the thickness of each of the insulating resin layers 2 a, 2 b, more preferably 80 to 95% of that. This will lead to not only a low resistance of the via, but also a narrow spacing between the front-sided wiring pattern 3 a and the rear-sided wiring pattern 3 b. This narrow spacing results in a smaller via.

The film 1 used in the flexible substrate 100 of the present invention generally has an insulating characteristic. Preferably, the film 1 is organic film such as a resin film. However, the film 1 is not limited if it has a heat-resisting characteristic, flexibility, smoothness and a low absorptivity. For example, film 1 is made of the materials selected from the group consisting polyethylene terephthalate (PET), polyphenylene sulfide (PPS), polyimide (PI), polyamide (PA), polyethylene naphthalate (PEN), polyether sulfone (PES), polyetherimide (PEI), polyarylate (PAR), polysulfone (PS), amorphous polyolefin (PO), polyamide-imide (PAI), liquid crystal polymer (LCP), modified polyphenylene ether (PPE), polybutylene terephthalate (PBT), polycarbonate (PC), polyether ether ketone (PEEK). The film made of such materials has a better heat-resisting characteristic and flexibility. Thus, by using of the film made of such materials, a flexible substrate suitable for folding and mounting in a spatially narrow area can be obtained, which in turn contributes to achievement of a smaller, lighter and thinner electronics device.

Among the above-mentioned materials, the polyamide is preferred in particular. The reason for this is that the polyamide has a high rigidity as well as a high heat-resisting characteristic. In particular, an aramid (i.e. aromatic polyamide) is preferred. The reason for this is that the aramid has an elastic characteristic even in the case where it is filmy, and therefore aramid is better in a handling ability, which in turn contributes to achievement of a thinner flexible substrate. Incidentally, among the aromatic polyamide, a para-aromatic polyamide is preferred because it is especially prominent in a high rigidity due to the linear structure of the main chain thereof and therefore a much thinner film can be obtained therefrom. Such thinner film is easy to be machined with the laser, allowing a formation of a very small via.

Each of the insulating resin layers 2 a, 2 b formed on both surface of the film 1 serves to house each of the wiring patterns 3 a, 3 b. In order to improve not only an adhesion between each of the insulating resin layers 2 a, 2 b and each of the wiring patterns 3 a, 3 b, but also an adhesion between substrates when multilayered, it is preferred that each of the insulating resin layers 3 a, 3 b has an adhesive property. Therefore, the material for the insulating resin layers 2 a, 2 b is at least one resin material selected from the group consisting of an epoxy resin, a polyimide resin, an acrylic resin and a modified resin thereof.

The wiring patterns 3 a, 3 b may be made of any materials if they have an electrically conductive property. For example, it is preferred that each of the wiring patterns 3 a, 3 b is made of the metal materials selected from the group consisting of a copper, a nickel, a gold and a silver. The thickness of each of the wiring patterns 3 a, 3 b can be altered as its usage, however, it is preferably about 3 to 18 μm. That is, it is preferred that each of the wiring patterns 3 a, 3 b is formed in such a manner that it is filmy. It is more preferred that the thickness of each of the wiring patterns is 3 to 12 μm in order to obtain a thinner flexible substrate having a high-density wiring.

The flexible substrate 100 of the present invention is characterized in that each of the wiring patterns 3 a, 3 b is embedded into each of the insulating resin layers 2 a, 2 b. In order to obtain such wiring patterns 3 a, 3 b, it is preferred a transferring technique is carried out. Concretely speaking as to a transferring technique, first, each carrier sheet on which each of wiring patterns 3 a, 3 b is preliminarily formed is prepared, and also a film 1 on both surfaces of which the insulating resin layers 2 a, 2 b are formed is prepared. It is preferred that the carrier sheet is made of an organic film (e.g. PET) or a metal foil (e.g. copper foil), and therefore the carrier sheet is a sheet-like member having about 25 to 200 μm in thickness. Subsequently, each carrier sheet is disposed on the film 1 in such a manner that each of the wiring patterns 3 a, 3 b formed on each carrier sheet is contacted with each of the insulating resin layers 2 a, 2 b, followed by pressing each of the wiring patterns 3 a, 3 b and each of the insulating resin layers 2 a, 2 b together. This causes each of the wiring patterns 3 a, 3 b to be embedded into each of the insulating resin layers 2 a, 2 b. In the case where each of the insulating resin layers 2 a, 2 b is made of a thermosetting resin, it is preferred that each of the insulating resin layers 2 a, 2 b is kept in a semi-curing state while each of the wiring patterns 3 a, 3 b is embedding. Finally, by removing the carrier sheet (including no wiring pattern), a sheet member in which each of the wiring patterns 3 a, 3 b is buried in each of the insulating resin layers 2 a, 2 b is obtained. In this case, it is preferred that the surface of each of the wiring patterns 3 a, 3 b is on the same (or approximately same) level as the surface of each of the insulating resin layers 2 a, 2 b. That is to say, the surface of each of the wiring patterns 3 a, 3 b is flush with the surface of each of the insulating resin layers 2 a, 2 b. This results in a better surface flatness of the obtained flexible substrate, which in turn leads to an advantage for a multilayering process of the substrates. The transferring technique gives a more fine-pitch wiring pattern than in the case of a wet etching process. For example, a line/space (L/S) of the wiring pattern for the case of the wet etching process is 40 μm/40 μm, while a line/space (L/S) of the wiring pattern for the case of the transferring technique is very fine 15 μm/15 μm (i.e. 30 μm pitch).

Each of the vias 4 is arranged between the front-sided wiring pattern 3 a and the rear-sided pattern 3 b in such a manner that each of the vias 4 is contacted with each of the wiring patterns 3 a, 3 b. Therefore, the via 4 serves to electrically connect the front-sided wiring pattern 3 a and the rear-side wiring pattern 3 b to each other. It is preferred that the via 4 is made of a conductive resin composition selected from the group consisting of a copper, a nickel and a silver.

Hereinafter, the preferred embodiments (I) to (V) of the flexible substrate of the present invention will be described:

According to a preferred embodiment (I), at least one of a passive element and an active element as well as a wiring connected thereto is formed on at least one of the front face and the rear face of the film. That is to say, at least one passive element and a wiring connected to the passive element are formed on at least one of the front face and the rear face of the film; at least one active element and a wiring connected to the active element are formed on at least one of the front face and the rear face of the film; or at least one of the combination of a passive element and an active element as well as a wiring connected to the combination.

FIG. 2 shows a cross-sectional view of the construction of the flexible substrate 100 including a passive element. The same elements as those of FIG. 1 are indicated by the same numbers in FIG. 2. What is different between FIG. 1 and FIG. 2 is that a condenser 5, a wiring 6 and a resistor 7 are arranged and that the via 4 is also arranged to connect among the front-sided wiring pattern 3 a, the rear-sided wiring pattern 3 b and the wiring 6 in FIG. 2, whereas there is no arrangement of a condenser, a wiring and a resistor in FIG. 1. In the embodiment shown in FIG. 2, the condenser 5 is electrically connected to the via 5 through the wiring 6, and the resistor 7 is also connected to the via 4 through the wiring 6. Incidentally, the wiring 6 may be an electrode wiring.

In this embodiment, it is possible to arrange an electronic circuit within the substrate by combing a variety of passive and active elements. Also, it is possible to mount the passive and active elements at a high-density with a short length of their wiring. This enables to minimize an adverse effect on the circuit, which effect is caused by a parasitic capacity between the wirings as well as an inductance. Further, the passive element and/or active element as well as the wiring are covered with the insulating resin layer, so that the adhesion among such elements, the wiring and the film is retained. This will lead to a sufficient flexibility of the flexible substrate 100. Also, the passive element and/or active element as well as the wiring are embedded in the insulating resin layer without being exposed to their surroundings on the surface of the substrate, so that the flat surface of the substrate is achieved. Therefore, a lamination process (i.e. stacking process) can be carried out without hindering the wiring. As a result, a high-density multilayer flexible substrate can be obtained.

According to a preferred embodiment (II), the passive element and the active element are respectively formed in such a manner that they are filmy. As used in this specification and claims that follows, the term “filmy” means that a thickness of the passive element and/or the active element is about 0.01 μm to 70 μm. A thin passive element and/or active element will lead to achievement of a thin flexible substrate having a sufficient flexibility.

According to a preferred embodiment (III), another passive element and/or active element are additionally formed.

FIG. 3 shows a cross-sectional view of the construction of the flexible substrate 120 of the present invention wherein another passive element and/or active element are additionally formed. The same elements as those of FIG. 2 are indicated by the same numbers in FIG. 3. What is different between FIG. 2 and FIG. 3 is that the resistor 8 is embedded in the insulating resin layer 2 b.

In this embodiment, a more high-performance electronic circuit composed of a variety of passive and active elements can be included within the flexible substrate. The passive and active elements can be mounted at a high-density with a short length of the wiring. This enables to minimize an adverse effect on the circuit, which effect is caused by a parasitic capacity between the wirings as well as an inductance.

It is preferred that the passive element is a element selected from the group consisting of a condenser composed of an inorganic dielectric, a resistor, an inductor and a combination thereof.

It is preferred that the inorganic dielectric consists of ATiO3 perovskite wherein “A” of “ATiO3” is at least one element selected from the group consisting of strontium (Sr), calcium (Ca), magnesium (Mg), barium (Ba), and lead (Pb). In the case where the inorganic dielectric consists of such element, a high dielectric constant of the condenser composed of such inorganic dielectric can be obtained. As a result, the electrostatic capacity per area becomes larger, which in turn leads to a smaller flexible substrate.

It is preferred that the resistor used as the passive element is made of materials selected from the group consisting of tantalum (Ta), titanium (Ti), nickel-chromium alloy (Ni—Cr), titanium-nickel alloy (Ti—Ni), tantalum nitride (TaN), chrome-silicon oxide (Cr—SiO), Tin-doped indium oxide (ITO), zinc oxide (ZnO), copper-aluminum oxide (CuAlO₂), strontium-copper oxide (SrCu₂O₂), aluminum-doped zinc oxide (AZO). In the case where the resistor is made of such materials, a high resistance of the resulting resistor can be obtained. As a result, the resistance per area becomes larger, which in turn leads to a smaller flexible substrate.

According to a preferred embodiment (IV), it is preferred that the active element is an organic semiconductor. In the case where an organic semiconductor is used as an active element, the resulting flexible substrate becomes lighter and thinner. It is possible to produce the organic semiconductor by a simple process such as a rotary press printing or an inkjet printer printing, instead of by a complicated process. Therefore, this will result in a decrease of the production cost.

In particular, it is preferred that the organic semiconductor is a pn junction solar cell. The reason for this is that a provision of the pn junction solar cell will give a high-density and a more high-performance electronic circuit. As a result, a module required no electric power supply can be obtained, for example. Further, compared with the prior solar cell made of an inorganic substance, there are some advantages to the pn junction solar cell. The pn junction will lead to a low-cost production of a lighter and thinner flexible substrate having a flexibility because a raw material of the pn junction solar cell is cheaper, and a large facility is not required for preparing the pn junction solar cell, and moreover the pn junction solar cell can be prepared just by applying an organic solution to the substrate.

Further, according to a preferred embodiment (IV), a multilayer substrate is produced by stacking the flexible substrates of the present invention to each other. FIG. 4 shows a cross-sectional view of the mulilayer flexible substrate 200 of the present invention. The multilayer flexible substrate 200 is composed of a first flexible substrate 101, a second flexible substrate 102 and a third flexible substrate 103. In this case, a variety of passive elements can be arranged within the multilayer flexible substrate 200, so that the area required for the surface-mounting as well as the number of the components is minimized, which in turn leads to a smaller electronics devise. Also, a high-density mounting of a variety of passive elements with a short length of the wiring helps to minimize an adverse effect on the circuit, which effect is caused by a parasitic capacity between the wirings as well as an inductance. As a result, a high-performance multilayer flexible substrate can be obtained.

Hereinafter, a process for producing a flexible substrate of the present invention will be described.

According to the present invention, a process for producing a flexible substrate composed of a film, an insulating resin layer and a wiring pattern, said process comprising the steps of:

-   -   (a) forming the insulating resin layer on each of a front face         of the film and a rear face of the film which face is opposite         to the front face wherein the insulating resin layer is thicker         than the film;     -   (b) making a through hole in the film and the insulating resin         layer;     -   (c) filling the through hole with a conductive resin         composition; and     -   (d) embedding a wiring pattern into each of the insulating resin         layers formed on the front face and the rear face of the film in         such a manner that the wiring pattern is electrically connected         to the conductive resin composition. According to the process of         the invention, it is possible to arrange the via at an optional         position. This allows an electric continuity at a desired         position of the wiring pattern, which will facilitate a wiring         design.

In the step (a), the insulating resin layers are formed on the front face of the film and the rear face opposite to the front face wherein each of the insulating resin layers is thicker than the film. For example, an epoxy resin, a polyimide resin, an acrylic resin or a modified resin thereof is used as a material of the insulating resin layer. Therefore, such resin material is supposed to be applied or supplied to the surface of the film. It is preferred that such resin material is applied or supplied by means of a dipping process, a roll coater process, a die coater process, a spray process or a curtain process. It is also preferred that the applied or supplied resin material is kept in a semi-curing state in spite of a drying process carried out. Accordingly, the obtained insulating resin layer is preferably subjected to a temperature of from 40 to 100° C.

Subsequently, in the step (b), the film and each of the insulating resin layers are perforated for the purpose of making a through hole. The diameter of the through hole is preferably 5 to 100 μm, more preferably 10 to 50 μm. A laser, a punch or a drill may be used to make the through hole.

In the step (c), the through hole is filled with a conductive resin composition. Prior to that, it is required to prepare the conductive resin composition in a paste state. Accordingly, it is preferred that the prepared conductive resin composition has a viscosity of from 10 to 300 Pa.s under a temperature of from 25 to 40° C. A screen printing process may be carried out to fill the through hole with a conductive resin composition. Incidentally, not applying only to filling the through hole with the conductive resin composition, through-hole plaiting may be carried out to provide a metal with the inner wall of the through-hole.

In the step (d), each of the wiring patterns 3 a, 3 b is embedded into each of the insulating resin layers 2 a, 2 b, and thereby each of the wiring patterns 3 a, 3 b is electrically connected to the conductive resin composition. Accordingly, the wiring patterns 3 a, 3 b are required to be located at such a position that each of the wiring patterns 3 a, 3 b is contacted with the conductive resin composition filled in the through-hole. In this step (c), preferably a transferring technique is employed. In this case, the wiring patterns formed preliminarily on a carrier sheet is transferred to the insulating resin layers obtained by the steps (a) to (c). In the case where the transferring technique is employed, it is possible to inspect the wiring pattern before transferring, so that only a good-quality wiring pattern can be embedded into the insulating resin layer. This will lead to a better process yield. As to the transferring technique, it is preferred that a carrier sheet is pressed toward the insulating resin layer under the condition of a temperature of 40° C. and a pressure of from 0.1 to 3 MPa. After the wiring pattern is embedded, it is preferred that a real curing of the insulating resin layer is carried out under the condition of a temperature of from 100 to 200° C. and a pressure of from 0.1 to 3 MPa.

In a preferred embodiment, the process of the present invention further comprises the step of forming at least one passive element and/or active element as well as the wiring connected thereto on at least one of the front face and the rear face of the film. In this case, the film used in the step (a) is supposed to be such a film that at least one passive element and/or active element as well as the wiring connected thereto is formed on at least one of the front face and rear face thereof. In the step (c), the wiring pattern on the film is supposed to be electrically connected to the conductive resin composition filled in the through-hole. The connection between the wiring and the conductive resin composition leads to the connection between the front-sided wiring pattern and the rear-sided wiring pattern. In this embodiment, an electronic circuit can be prepared by combing a variety of passive and active elements. Also, it is possible to mount the passive and active elements at a high-density with a short length of their wiring. This enables to minimize an adverse effect on the circuit, which effect is caused by a parasitic capacity between the wirings as well as an inductance. In the process of the present invention, at least one passive element and/or active element as well as the wiring connected thereto are covered with the insulating resin layer, and thereby a high adhesion strength among the film, the passive element, the active element and the wiring is maintained, and therefore the flexible substrate having a better flexibility can be obtained.

At least one passive element and/or active element as well as the wiring connected thereto may be formed by means of a sputtering method (i.e. sputtering technique), a vacuum deposition method, or an ion plating method. In this case, even at low temperature, the passive element, the active element and the wiring can be formed from a material having a high-melting point in such a manner that they are filmy. This will lead to a better adhesion among the film, the passive element, the active element and the wiring. Therefore, the passive element, the active element and the wiring can be formed with no damage to them.

Likewise, a screen printing method (i.e. screen printing technique), a metal mask printing method, or a drawing method may be also employed. Even by means of these methods, it is possible to form a film at a low temperature. In this case, the passive element, the active element and the wiring can be formed not only with no damage to them but also at a low cost.

In addition to transferring the wiring, it is possible to transfer the passive element and/or the active element to the insulating resin layer. In this case, a wiring as well as a passive element and/or an active element is preliminarily formed on the carrier, and thereafter the wiring as well as the passive element and/or the active element is embedded into each of the insulating resin layers formed on the front face and the rear face of the film.

Hereinabove, the process for producing a flexible substrate has been described. By carrying out such process repeatedly, the multilayer flexible substrate (see FIG. 4) can be obtained.

Turning now to FIG. 5, an example of the production process of a flexible substrate 100 according to the present invention will be hereinafter described.

First, an insulating material such as an epoxy-based thermosetting resin composition is dissolved in an aromatic solvent (e.g. toluene or xylene), a ketone-based solvent (e.g. methyl ethyl ketone or acetone), an alcohol-based solvent (e.g. methanol or ethanol), or a polar solvent (e.g. dimethylformamide or dimethylacetamide), and thereby an insulating material like a varnish is prepared. Subsequently, such insulating material is applied to the both surfaces of the organic film 1 made of a polyamide or a polyimide by means of the process such as a dipping process, a roll coater process, a die coater, a spray process or a curtain process. As a result, a sheet member in which the insulating resin layers 2 a, 2 b are formed is obtained (see FIG. 5 (a)). Preferably, it is needed that the obtained insulating resin layers 2 a, 2 b are kept in a semi-curing state.

Subsequently, for the purpose of making the through-hole 13, the sheet member is perforated by means of a carbon dioxide gas laser or a UV laser (see FIG. 5(b)). Subsequently, the through-hole is filled with the conductive resin composition 14 (see FIG. 5(c)), followed by embedding each of the wiring patterns 3 a, 3 b into each of the insulating resin layers 2 a, 2 b by means of the transferring technique. In this case, it is preferred that the surface of each of the wiring patterns 3 a, 3 b is on the same level as the surface of each of the insulating resin layers 2 a, 2 b. That is to say, the surface of each of the wiring patterns 3 a, 3 b is flush with the surface of each of the insulating resin layers 2 a, 2 b. When each of the wiring patterns 3 a, 3 b is embedded into each of the insulating resin layers 2 a, 2 b, a portion of the front-sided wiring pattern 3 a and a portion of the rear-sided wiring pattern 3 b is connected to the conductive resin composition 14. This results in a formation of the via 4 (see FIG. 5(d)). Finally, by carrying out the real curing of the insulating resin layers 2 a, 2 b, the flexible substrate 100 of the present invention can be obtained.

Turning now to FIG. 6, an example of the production process of a flexible substrate 110 including a passive element will be hereinafter described.

First, a condenser 5 and a wiring 6 are formed on one of the front face and the rear face of the organic film 1 such as an aramid film or a polyimide film (see FIG. 6(a)). The wiring 6 is formed in such a manner that a part of the wiring 6 is located at the position of the via to be formed. By means of a sputtering method, a vacuum deposition method or an ion plating method, the condenser 5 and the wiring 6 may be formed in such a manner that they are thin-filmy (i.e. about 0.01 to 1 μm in film thickness). Alternatively, by means of a screen printing method, a metal mask printing method or a drawing method, the condenser 5 and the wiring 6 may be formed in such a manner that they are thick-filmy (i.e. about 1 to 70 μm in film thickness).

Although the condenser 5 is formed as a passive element in the production process shown in FIG. 6, not applying only to that, an active element may be formed on one of the front face and the rear face of the film 1. In this case, it is preferred that an active element is an organic semiconductor. It is further preferred that the organic semiconductor is a pn junction solar cell.

Subsequently, an insulating material such as an epoxy-based thermosetting resin composition is dissolved in an aromatic solvent (e.g. toluene or xylene), a ketone-based solvent (e.g. methyl ethyl ketone or acetone), an alcohol-based solvent (e.g. methanol or ethanol), or a polar solvent (e.g. dimethylformamide or dimethylacetamide), and thereby an insulating material like a varnish is obtained. Subsequently, by means of the process such as a dipping process, a roll coater process, a die coater, a spray process or a curtain process, the above insulating material is applied to the both surfaces of the film 1 in which the condenser 5 and wiring 6 are already formed thereon. As a result, a sheet member in which the insulating resin layers 2 a, 2 b are formed is obtained (see FIG. 6 (b)). Preferably, it is needed that the obtained insulating resin layers 2 a, 2 b are kept in a semi-curing state.

Subsequently, for the purpose of making the through-hole 13, the sheet member is perforated by means of a carbon dioxide gas laser or a UV laser (see FIG. 6(c)). Subsequently, the through-hole is filled with the conductive resin composition 14 (see FIG. 6(d)), and therefore the filled conductive resin composition 14 is connected to the wire 6. After that, each of the wiring patterns 3 a, 3 b is embedded into each of the insulating resin layers 2 a, 2 b by means of the transferring technique. In this case, it is preferred that the surface of each of the wiring patterns 3 a, 3 b is on the same level as the surface of each of the insulating resin layers 2 a, 2 b. That is to say, the surface of each of the wiring patterns 3 a, 3 b is flush with the surface of each of the insulating resin layers 2 a, 2 b. When each of the wiring patterns 3 a, 3 b is embedded into each of the insulating resin layers 2 a, 2 b, a portion of the front-sided wiring pattern 3 a and a portion of the rear-sided wiring pattern 3 b is connected to the conductive resin composition 14. As a result of that, this results in a formation of the via 4 (see FIG. 5(d)). Finally, by carrying out the real curing of the insulating resin layers 2 a, 2 b, the flexible substrate 110 including the passive element can be obtained.

Next, according to the present invention, an example of the production process of a multilayer flexible substrate will be hereinafter described.

The multilayer flexible substrate of the present invention includes the flexible substrate obtained by the above-mentioned production process as a fundamental structure. Accordingly, first, a plurality of substrates in which the wiring patterns are buried in the insulating resin layers are prepared by means of the above-mentioned production process. The prepared insulating resin layers are kept in a semi-curing state so that they are not completely cured. Subsequently, a plurality of the prepared substrates are stacked to each other with adjustment of the position, so that a precursor of the multilayer flexible substrate is obtained. After that, the insulating resin layers of the precursor are cured as a whole by means of a roller type heat-pressure device. On this occasion, the precursor of the multilayer flexible substrate is supposed to be pressed by means of the clearance gap of the roller, and concurrently the insulating resin layers are supposed to be heated to be melted, followed by the curing thereof. As a result of that, the precursor is integrated to form a multilayer flexible substrate. In this process, a reel-to-reel process or a roll-to-roll process may be employed, and thereby it is possible to produce a multilayer flexible substrate more simply than in the case where a parallel-flat board type hot-press of the prior art is employed. The reel-to-reel process or the roll-to-roll process is better in producibility because these processes enable to continuously produce a flexible substrate from an long substrate, and therefore these processes can produce a multilayer flexible substrate at low cost.

EXAMPLE

According to examples (1) to (4), the experiments about the flexible substrate and the process for producing the same were performed.

First, according to examples (1) and (2), the experiments concerning a flexing life of the flexible substrate of the present invention were performed.

Example 1

(Film Material)

A film (organic film) used in this example is shown in Table 1. TABLE 1 No. FILM NAME OF COMMODITY (MANUFACTURER) 1a ARAMID ┌MICTRON┘ (TORAY Co., Ltd.) 1b ARAMID ┌ARAMICA┘ (TEIJIN ADVANCED FILM Co., Ltd.) 1c PI ┌KAPTON┘ (DUPONT-TORAY Co., Ltd.) 1d PI ┌UPILEX┘ (UBE INDUSTRIES Co. Ltd.) 1e PEN ┌TEONEX┘ (TEIJIN-DUPONT FILM Co. Ltd.) 1f PET ┌TETRON┘ (TEIJIN-DUPONT FILM Co. Ltd.) 1g PPS ┌TORELINA┘ (TORAY Co., Ltd) 1h PA ┌HARDEN┘ (TOYOBO Co., Ltd) 1i PC ┌PANLITE┘ (TEIJIN CHEMICALS Co., Ltd) 1j PES ┌SUMILITE FS-13OO┘ (SUMITOMO BAKELITE Co., Ltd) 1k PEI ┌SUPERIO UT┘ (MITSUBISHI PLASTICS, Inc.) 1l PPE ┌DIANIUM┘ (MITSUBISHI PLASTICS, Inc.) 1m PEEK ┌SUMILITE FS-11OOC┘ (SUMITOMO BAKELITE Co., Ltd) (Preparation of the Substrate Used for Measuring Flexing Life)

By applying an epoxy-base thermosetting resin layer to both surfaces of the film with a roll coater process, the insulating resin layers were formed. Subsequently, the wiring patterns were embedded into the insulating resin layers.

Prior to embedding of the wiring patterns, a thin peel-apart layer (consisting of a nickel-phosphorus alloy) was prepared on both surfaces of the electrolytic copper foil having a thickness of 70 μm (which foil served as a supporting member of the wiring pattern), and then another copper foil (12 μm in thickness) was formed on the peel-apart layer by means of an electroplating technique. After that, a wiring pattern was formed by superposing a dry film resist on the formed copper foil, followed by carrying out a light exposure, a development, an etching, and a removing of the resist in series.

Subsequently, the supporting member having the wiring pattern was superposed on the insulating resin layers formed on the front face and the rear face of the film with adjustment of the position, and therefore the wiring pattern of the supporting member was embedded into the insulating resin layer by heating to a temperature of 60° C. as well as pressing at the pressure of 3 MPa for 5 minutes. Subsequently, after cooling, the only supporting member was peeled off, followed by carrying out a real curing of the insulating resin layer by heating it for an hour under the condition of 140° C. and 5 MPa. In this way, a substrate (i.e. sample substrate), which is regarded as a base member for the flexible substrate, was obtained. The specification of the obtained substrate is shown in the following Table 2. TABLE 2 MODULUS OF THICKNESS(μm) ELASTICITY (GPa) INSULATING INSULATING ORGANIC WIRING RESIN LAYER RESIN No. FILM PATTERN (ON ONE SURFACE OF FILM) FILM SUBSTRATE LAYER FILM 1a ARAMID 12 15 12 42 0.8 13 1b ARAMID 12 15 12 42 0.8 15 1c PI 12 15 12 42 0.8 3.2 1d PI 12 15 12 42 0.8 8 1e PEN 12 15 12 42 0.8 6.5 1f PET 12 15 12 42 0.8 5.5 1g PPS 12 15 12 42 0.8 3.9 1h PA 12 15 12 42 0.8 1.5 1i PC 12 15 12 42 0.8 2.3 1j PES 12 15 12 42 0.8 2 1k PEI 12 15 12 42 0.8 2.8 1l PPE 12 15 12 42 0.8 1.7 1m PEEK 12 15 12 42 0.8 3.2 (Measurement of Flexing Life)

Based on the technique of IPC-240C and JIS-C5016, the flexing life for various types of the sample substrates was measured.

Prior to the measurement, the sample substrate was fixed between two flat plates that respectively opposed at a certain distance in such a manner that the sample substrate was folded at 180 degree so as to achieve a constant curvature. Subsequently, the two flat plates were moved to each other in parallel at a predetermined speed and stroke. That is to say, the sample substrate was repeatedly slid and moved to each other so that a reciprocating motion thereof was carried out. During that, the direct current resistance of the wiring patterns located at the curved inner surface of the sample substrate was monitored. The flexing life was regarded as a cycle number of the reciprocating motion wherein the resistance was increased by 80% compared with the initial resistance. By way of comparison, the flexing life of the copper foil used for the wiring pattern (i.e. copper foil having a thickness of 12 μm, and which was formed by electroplating technique) was also measured with a similar method to the above example.

(Result)

The result of the example is shown in FIG. 7. FIG. 7 is a graph that shows the modulus of elongation of the film versus the number of flexing (=flexing life) at a room temperature. A comparative example showed that a rupture occurred at 800 cycles of the reciprocating motion. Considering that along with reference to FIG. 7, it is understood that the substrate, which is considered as a base member for the flexible substrate of the present invention, has a preferable flexing life irrespective of the modulus of the elongation. The reason for this is that, due to the wiring patterns buried in insulating resin layers, the applied stress tends to be dispersed by the insulating resin layer that retains the wiring pattern, which in turn prevents the development of the microcrack that may occurr in the wiring pattern.

Example 2

(Preparation of the Substrate Used for Measuring Flexing Life)

In this example, by using of the method similar to the above example (1), various types of the sample substrates were prepared in such a manner that a ratio of insulating resin layer thickness/film thickness is diversely changed. The specification of the prepared substrate is shown in the following Table 3. TABLE 3 THICKNESS(μm) MODULUS OF INSULATING INSULATING ELASTICITY (GPa) RESIN LAYER ORGANIC WIRING RESIN LAYER INSULATING THICKNESS/ No. FILM PATTERN (ON ONE SURFACE OF FILM) FILM SUBSTRATE RESIN LAYER FILM FILM THICKNESS 2a ARAMID 3 12 4 28 0.8 13 3.0 2b ARAMID 3 11 6.5 28.5 0.8 13 1.7 2c ARAMID 3 9.5 9 28 0.8 13 1.1 2d ARAMID 3 8 12 28 0.8 13 0.7 2e ARAMID 3 6 16 28 0.8 13 0.4 2f ARAMID 3 6 9 21 0.8 13 0.7 2g ARAMID 3 12 9 33 0.8 13 1.3 (Test Condition)

All of the films used in this example were aramid film (“MICTRON” manufactured by TORAY Co., Ltd.) The sample substrates 2 a to 2 e were approximately same in thickness thereof, and the sample substrates 2 c, 2 f, and 2 g were same in thickness of the film. The flexing life of such sample substrates were measured by using of the method similar to the above example (1). As another test condition, the speed (i.e. frequency) was 25 Hz, the stroke was 25 mm, and the curvature radius was 2 mm, 4 mm and 8 mm.

(Result)

The results of the example (2) are shown in the FIGS. 8 and 9. FIG. 8 is a graph that shows a curvature radius versus a flexing number (=flexing life). FIG. 9 is a graph that shows a ratio of an insulating resin layer thickness to a film thickness (=insulating resin layer thickness/film thickness) versus a flexing number (=flexing life). With reference to these graphs, it is understood that the flexible life of the sample substrate is better in the case where the insulating resin layer is thicker than the film, and that the smaller a curvature radius becomes, the more distinctive a feature concerning the flexible life becomes. The reason for this is that the stress applied on the wiring pattern and the film is supposed to be alleviated by the insulating resin layer having a low modulus of elasticity.

Next, in the following examples (3) to (5), a flexible substrate and multilayer substrate were produced by carrying out the production process of the present invention.

Example 3

(Production of the Flexible Substrate of the Present Invention)

The film used in the example (3) was aramid film (“MICTRON” manufactured by TORAY Co., Ltd.) having 4 μm in thickness. By means of a dipping process, an epoxy-based thermosetting resin was applied to the both surfaces of the film to prepared an aramid sheet with the epoxy resin layers thereon. The application was carried out in such a manner that the predetermined thickness of the epoxy resin layers was obtained. After that, the epoxy resin layers were dried in such a manner that they were kept in a semi-curing state. By means of a heat press, a PEN film was superposed on the upper surface of the aramid sheet (in which epoxy resin layers were formed thereon) having 9 μm in thickness under such a condition of a temperature of 40° C. and a pressure of 0.5 MPa that did not cause the epoxy resin layer to be melted. Subsequently, by means of the UV-YAG laser, a plurality of the through-holes, which were respectively 50 μm in diameter, were formed in the aramid sheet in which the epoxy resin layers were formed thereon, followed by superposing the PEN film on one of the epoxy resin layers. Subsequently, by means of the printing method, through-holes were filled with a conductive resin composition and then dried. In terms of a filling efficiency for a small hole, the conductive resin composition was impasted. Therefore, the three rolls were used to knead a copper powder of 70 wt % (1 μm in an average diameter), bisphenol A type epoxy resin of 10 wt % (serving as a resin component), amineadduct curing agent of 3 wt % (serving as a curing agent for epoxy resin) and butylcarbitolacetate of 17 wt %, based on the total weight of the conductive resin composition. As to the filling, the PEN film was used as a mask. And also, the existing screen printer was employed. That is to say, by means of a polyurethane squeegee, the conductive resin layer paste was imprinted from the surface of the substrate.

Subsequently, the wiring pattern was formed on the aramid sheet in which the epoxy resin layer were already formed thereon. Prior to forming the wiring pattern, two complex copper foils (“PIRABLE” manufactured by FURUKAWA CIRCUIT FOIL Co., Ltd.) were prepared wherein a copper having a predetermined thickness (5 μm, 9 μm, 12 μm) were formed on one surface of the copper foil (70 μm in thickness) by means of an electroplating technique. After that, the wiring pattern was formed by superposing a dry film resist on the surface of the copper plating layer, followed by carrying out a light exposure, a development, an etching, and a removing of the resist in series. In this case, a half-etching process may be carried out in order to obtain a various thicknesses of the wiring pattern.

Subsequently, the PEN film was peeled off from the aramid sheet (in which epoxy resin layers were already formed thereon), followed by embedding the wiring pattern obtained from the copper foil into the epoxy resin layer at a pressure of 3 MPa, and then subjected to a temperature of 80° C. for 5 minutes. After cooling, the copper foil serving as a carrier sheet was peeled off. Finally, the real curing of the insulating resin layers was carried out by heating it for an hour under the condition of 180° C. and 5 MPa, In this way, a flexible substrate was obtained.

(Measurement of the Value of the Specific Resistance of Via)

The specific resistance of vias (i.e. inner via) formed in the flexible substrate was measured. First, by means of a four-terminal measurement, the resistance of the via was measured through the wiring pattern formed on the copper foil with the condition of series-connected 500 vias, followed by subtracting the resistance of the copper foil from the measured resistance. The specific resistance for the via was calculated based on the filling volume, which volume is obtained from the thickness of the substrate as well as the diameter of the hole.

Considering that the specific resistance for the metal copper particle used in the example was 1.7×10⁻⁶ Ω cm, the judging standard was as follows: the specific resistance of less than 10 times that of the copper particle was considered as “excellent”, while the specific resistance of more than 10 times and less than 100 times that of the copper particle was considered as “better”, and the specific resistance of more than 100 times that of the copper particle was considered as “defective”

The result is shown in Table 4. According to Table 4, it was confirmed that an electrical connection concerning any via was established. In particular, in the case where the thickness of the wiring pattern was more than and equal to 80% of the thickness of the insulating resin layer (i.e. epoxy resin layer), the specific resistance of the via is less that that of the copper particle, and therefore a low resistance of the via can be obtained under that condition. TABLE 4 RATIO OF WIRING THICKNESS THICKNESS THICKNESS SPECIFIC SAMPLE OF WIRING OF RESIN LAYER TO RESIN LAYER RESISTANCE No. (μm) (μm) THICKNESS (%) (Ω · cm) JUDGEMENT 1 2 5 40 9 × 10⁻⁵ GOOD 2 3 5 60 5 × 10⁻⁵ GOOD 3 4 5 80 6 × 10⁻⁶ EXCELLENT 4 5 5 100 5 × 10⁻⁶ EXCELLENT 5 2 10 20 9 × 10⁻⁵ GOOD 6 4 10 40 9 × 10⁻⁵ GOOD 7 6 10 60 4 × 10⁻⁵ GOOD 8 8 10 80 4 × 10⁻⁶ EXCELLENT 9 10 10 100 4 × 10⁻⁶ EXCELLENT

Example 4

(Production of the Flexible Substrate Including a Passive Element)

The film used in the example (3) was aramid film (“MICTRON” manufactured by TORAY Co., Ltd.) having 4 μm in thickness. By means of a sputtering technique, titanium layer (0.05 μm in thickness) and platinum layer (0.2 μm in thickness) were in turn formed on the aramid film. Subsequently, by means of a photolithography technique, a patterning process for forming a predetermined shape was carried out in order to form a lower electrode of the condenser. After that, a strontium titanate layer (0.1 μm in thickness) was formed by means of a RF sputtering technique at 400° C., followed by patterning it to form a dielectric layer by means of a photolithography technique. Subsequently, an upper electrode was formed on the dielectric layer in the same way as the lower electrode, so that a condenser was formed on the aramid sheet. A crossover area between the upper electrode and the lower electrode was 100 μm×100 μm. Each electrode extended toward outer side of the edge of the dielectric layer, so that the vias to be formed was supposed to penetrate through the electrode. Furthermore, by means of a sputtering technique, a titanium layer (0.03 μm in thickness) was formed on the aramid film, followed by carrying out a patterning it to form a resistor film (100 μm×100 μm) by means of a photolithography technique. After that, by means of a sputtering technique and a plating technique, a pair of electrode wirings (consisting of copper, 20 μm in thickness) was formed on both edges of the resistor film in such a manner that the resistor film (concretely the resistor film of 100 μm wide×100 μm long) was overlaid with the electrode wiring. Each electrode wiring extended toward outer side of the edge of the resistor film, so that vias to be formed was supposed to penetrate through the electrode wiring.

Subsequently, by means of a dipping process, an epoxy-based thermosetting resin was applied to the both surfaces of the film in which the passive element was already formed thereon. The application was carried out in such a manner that the predetermined thickness (10 μm) of the epoxy resin layers was obtained. As a result, the aramid sheet in which epoxy resin layers were formed thereon was obtained. After that, the epoxy resin layers were dried in such a manner that they were kept in a semi-curing state. By means of a heat press, a PEN film was superposed on the upper surface of the aramid sheet having 9 μm in thickness under such a condition of a temperature of 40° C. and a pressure of 0.5 MPa that did not allow the epoxy resin layer to be melted. Subsequently, by means of a UV-YAG laser, a plurality of the through-holes, which were respectively 50 μm in diameter, were formed in the aramid sheet, followed by superposing the PEN film on one of the epoxy resin layers. Subsequently, by means of the printing method, through-holes were filled with a conductive resin composition and then dried. In terms of a filling efficiency for a small hole, the conductive resin composition was impasted. Therefore, the three rolls were used to knead a copper powder of 70 wt % (1 μm in an average diameter), bisphenol A type epoxy resin of 10 wt % (serving as a resin component), amineadduct curing agent of 3 wt % (serving as a curing agent for epoxy resin) and butylcarbitolacetate of 17 wt %, based on the total weight of the conductive resin composition. As to the filling, the PEN filim was used as a mask. And also, the existing screen printer was employed. That is to say, by means of a polyurethane squeegee, the conductive resin layer paste was imprinted from the surface of the substrate.

Subsequently, the wiring pattern was formed on the aramid sheet in which the epoxy resin layer were already formed thereon and the passive element was already included. Prior to forming the wiring pattern, two complex copper foils (“PIRABLE” manufactured by FURUKAWA CIRCUIT FOIL Co., Ltd.) were prepared wherein a copper having a predetermined thickness (5 μm) were formed on one surface of the copper foil (thickness of which is 70 μm) by means of an electroplating technique. After that, the wiring pattern was formed by superposing a dry film resist on the surface of the copper plating layer, followed by carrying out a light exposure, a development, an etching, and a removing of the resist in series.

Subsequently, the PEN film was peeled off from the aramid sheet (in which epoxy resin layers were formed thereon), the wiring pattern obtained from the copper foil into the epoxy resin layer at a pressure of 3 MPa, and then subjected to a temperature of 80° C. for 5 minutes. After cooling, the copper foil serving as a carrier sheet was peeled off. Finally, the real curing of the insulating resin layers was carried out by heating it for an hour under the condition of 180° C. and 5 MPa, In this way, a flexible substrate including the passive element was obtained.

(Measurement of the Resistance of the Passive Element)

The electric characteristic of a condenser and a resistor was measured between before and after they are included in the flexible substrate. As to the measurement of the condenser before included, the capacity of 2.2 fF was obtained with a measuring signal of 1 kHz Likewise, as to the measurement of the resistor before included, the resistance of 100 Ω was obtained with a measuring signal of 1 kHz. As to the measurement of the condenser as well as the resistor after embedded into an insulating resin layer (i.e. epoxy resin layer), it resulted in approximately the same findings as those before included. Therefore, the passive elements such as a condenser and a resistor remain nearly unaffected even if they are included in the flexible substrate of the present invention.

Example 5

(Production of the Multilayer Flexible Substrate of the Present Invention)

In the example (5), a multilayer flexible substrate was produced by using of the flexible substrate as obtained in examples (2) or (3). First, three substrates were prepared, each of which the wiring patterns were buried in the insulating resin layers formed on both surfaces of the film. The insulating resin layers were kept in a semi-curing state so that they were not completely cured. Subsequently, three substrates were stacked to each other with adjustment of the position, so that a precursor of the multilayer flexible substrate (i.e. flexible four-layer substrate) was obtained. After that, the insulating resin layers of the precursor were cured as a whole by means of a roller type heat-pressure device. By adjusting clearance gap of a pair of rollers of the heat-pressure device, a nip pressure was applied to the precursor, which pressure corresponded to 5 MPa employed in the case of a parallel-flat board type press device of the prior art. The temperature of the roller was 200° C. Upon passing through the clearance gap of a pair of rollers, the precursor was pressed as a whole, and concurrently the insulating resin layers were heated to be melted and cured. As a result of that, the precursor was integrated to produce a multilayer flexible substrate in which three flexible substrates were laminated to each other.

INDUSTRIAL APPLICABILITY

The flexible substrate and the multilayer substrate according to the present invention are better in a high-density and a flexing life regardless of a thinness thereof. Therefore, they will contribute to achievement of a smaller, lighter and thinner electronics device.

CROSS REFERENCE TO RELATED PATENT APPLICATION

The present application claims the right of priority of Japanese Application No. 2004-079847 (filed Mar. 19, 2004, the title of the invention: “FLEXIBLE SUBSTRATE, FLEXIBLE MUTILAYER SUBSTRATE, AND PROCESS FOR PRODUCING THE SAME”), the disclosure of which is all incorporated herein by reference. 

1. A flexible substrate comprising: (i) a film; (ii) an insulating resin layer formed on each of a front face of said film and a rear face of said film which face is opposite to said front face; (iii) a front-sided wiring pattern embedded in the insulating resin layer formed on said front face of said film, and a rear-sided wiring pattern embedded in the insulating resin layer formed on said rear face of said film; and (iv) a via which is located between a front-sided wiring pattern and a rear-sided wiring pattern and serves to electrically connect between said front-sided wiring pattern and said rear-sided wiring pattern; wherein said insulating resin layer formed on each of said front face and said rear face of said film is thicker than said film.
 2. The flexible substrate according to claim 1, wherein a ratio of an insulating resin layer thickness to a film thickness is 1.2 to
 6. 3. The flexible substrate according to claim 1, wherein a thickness of each of said front-sided wiring pattern and said rear-sided wiring pattern is 80 to 95% of a thickness of said insulating resin layer formed on each of said front face of said film and said rear face of said film.
 4. The flexible substrate according to claim 1, wherein said via is made of a conductive resin composition.
 5. The flexible substrate according to claim 1, wherein at least one of a passive element and an active element as well as a wiring connected thereto is formed on at least one of said front face and said rear face of said film, and said wiring and said via are electrically connected to each other.
 6. The flexible substrate according to claim 5, wherein said passive element and/or said active element are formed in such a manner that they are filmy.
 7. The flexible substrate according to claim 5, wherein said passive element is selected from the group consisting of a condenser composed of an inorganic dielectric, a resistor, an inductor and a combination thereof.
 8. The flexible substrate according to claim 1, wherein said film is made of an aramid or a polyimide.
 9. The flexible substrate according to claim 1, wherein said insulating resin layer is made of at least one resin selected from the group consisting of an epoxy resin, a polyimide resin, an acrylic resin and a modified resin thereof.
 10. The flexible substrate according to claim 5, wherein said active element is an organic semiconductor.
 11. The flexible substrate according to claim 10, wherein said organic semiconductor is a pn junction solar cell.
 12. A multilayer flexible substrate in which a plurality of flexible substrates are laminated, wherein at least one of said flexible substrates is the flexible substrate according to claim
 1. 13. A process for producing a flexible substrate comprising a film, an insulating resin layer and a wiring pattern, said process comprising the steps of: (a) forming the insulating resin layer on each of a front face of said film and a rear face of said film which face is opposite to said front face wherein said insulating resin layer is thicker than said film; (b) making a through hole in said film and said insulating resin layer; (c) filling said through hole with a conductive resin composition; and (d) embedding a wiring pattern into each of the insulating resin layers formed on said front face and said rear face of said film in such a manner that said wiring pattern is electrically connected to said conductive resin composition.
 14. The process according to 13, wherein a ratio of an insulating resin layer thickness to a film thickness is 1.2 to
 6. 15. The process according to claim 13, wherein at least one of a passive element and an active element as well as a wiring connected thereto are formed on at least one of said front face and said rear face of said film used in said step (a), and in said step (c), said wiring is electrically connected to said conductive resin composition filled in said through hole.
 16. The process according to claim 15, wherein said passive element and/or said active element are formed by means of a sputtering technique or a screen printing technique.
 17. The process according to claim 13, wherein said wiring pattern is embedded in said insulating resin layer by transferring a preliminarily formed wiring pattern to said insulating resin layer.
 18. The process according to claim 13, wherein said wiring pattern as well as said passive element and/or active element is embedded in said insulating resin layer by transferring a preliminarily formed wiring pattern as well as a preliminarily formed passive element and/or active element to said insulating resin layer. 